As a visible light communication technology for conveying information by means of an image or movie displayed on a display or the like, there has been a known system in which a special two-dimensional code (QR code) or the like is displayed on part of a displayed image so that the camera of a mobile phone etc., photographs the two-dimensional code to receive information.
For example, Patent Document 1 discloses a configuration which, when an anomaly is detected in a vehicle navigation system, enables visual check of fault diagnosis data by generating fault diagnosis data including an anomalous signal that indicates the anomaly, by converting the fault diagnosis data into a symbol code (bar code or two-dimensional code) to be displayed on a display apparatus, and by reading the displayed symbol code by means of a user's mobile terminal.
On the other hand, Patent Document 2 discloses a configuration that enables acquisition of various kinds of contents by reading a two-dimensional code printed on a display medium by means of a terminal device and by accessing a server apparatus specified by the data from the read two-dimensional code and by providing images, sound and other contents.
In the visible light communication technologies described above, there is the problem that if, for example, a two-dimensional code is displayed on a large-scale display used on the street, the two-dimensional code will occupy part of the image (public image) such as an advertisement etc. displayed for the public, the aesthetic appearance will be spoiled.
As a means for dealing with this problem, there is a technology called secure display, which is comprised of a display apparatus that displays a public image and an image (private image) to be provided for only special users in a time-division manner and a light shutter that makes light pass therethrough in synchronization with the display timing of the private image.
In the secure display technique, a public image provided to unspecified people, a private image provided to only specific users, and a reverse image of the private image are displayed in a time division manner in one frame by the display device and the optical shutter present in front of the users is switched to a transmission state in synchronization with display timing for the private image. With this arrangement, the user who uses the light shutter can view the private image alone whereas unspecified people (the public) who use no light shutter see the public image only because the private image is superimposed with the reversal image to produce a half-tone (gray) image.
Use of this secure display technology makes it possible to provide a variety of information using private images for the users who use the light shutter while providing public images for unspecified people without spoiling the aesthetic appearance in the public image.
However, in the secure display technology, it is necessary to use the light shutter in order to view the private image as described above. Accordingly, users who can enjoy the private image are limited. In typical visual light communications, it is desired that various kinds of information can be provided in a simpler manner for a large number of users by use of private images.
For example, when an advertisement etc. is displayed as the public image while the URL (Uniform Resource Locator) of the server equipment that provides the detail information on the product displayed by the public image is displayed by a two-dimensional code or the like, a system that requires use of the light shutter is less convenient.
Further, in the secure display technology, in order to cause the light shutter to pass through only the private image displayed on the display apparatus therethrough and block the public image and the reversal image, it is necessary to transmit a synchronization signal that synchronizes with the display of the private image, from the display apparatus to the light shutter.
However, in visual light communications, it is not always possible for the user to receive the synchronization signal because in such a case various kinds of information are acquired from the image displayed on a large-scale display that is located far away, for example. It is therefore desired to be able to provide information to users simply without using any synchronization signal, unlike the above-described secure display technology.
Further, in the secure display technology, since the public image, private image and reversal image are switched and displayed at relatively high speed, the imaging device for photographing the display image is also required to have a relatively high-speed image processing capability in order to extract the private image alone from the displayed image.
However, since the system that needs a special device to acquire the private image as described is not desirable, it is preferred that even a device that has a relatively low image processing capability as, for example a built-in camera of a mobile phone, acquire the private image.